Tuberculosis (TB) is a major cause of infectious mortality. According to a recent WHO report, the number of deaths attributed to TB was a larger number in 1995 than in any other year in history (Moran, N. 1996. WHO Issues Another Gloomy Report. Nature Medicine 4:377). Tuberculosis remains widespread worldwide and constitutes a major health problem particularly in developing countries. One third of the total world's population (nearly two billion people) is infected with Mycobacterium tuberculosis out of which 5 to 10% develop the disease. TB causes more than 3 million deaths per year and recently WHO has predicted that 30 million people will die of TB in the next ten years (Joint International Union Against Tuberculosis and World Health Organization Study Group. Tubercl 63:157-169, 1982).
Tuberculosis is caused by a gram positive acid fast bacterium Mycobacterium tuberculosis or M. bovis, which are the tubercle bacilli of the family of Mycobacteriaceae. M. bovis is a species which causes tuberculosis in cattle and can be transmitted to humans and other animals in which it causes tuberculosis. At present nearly all tuberculosis in humans is caused by Mycobacterium tuberculosis. Infections occasionally result from other species of mycobacteria that are common environmental saprophytes. These species have been collectively termed as MOTT (Mycobacteria other than typical tubercle), environmental or tuberculoid bacilli. The difference between the two infections is that infection with Mycobacterium tuberculosis is always transmitted from host to host. In contrast, human beings infected with other mycobacteria rarely transmit the disease.
Hence the essential component of any tuberculosis control program is containment of the disease. Identification of infected individuals, especially those most likely to transmit viable bacilli, comes as a first priority in strategies for tuberculosis control. Early and timely diagnosis of tuberculosis is essential for identifying individuals carrying the bacilli. Therefore a need has arisen for a method of diagnosis of tuberculosis which is rapid, sensitive and specific. Routine diagnostic methods used for identification of Mycobacterium tuberculosis includes acid fast smear test in clinical samples like sputum, tests based on growth of bacilli in specific media and differential biochemical tests. The culture of mycobacteria from clinical samples is the most reliable and provides for definite diagnosis of tuberculosis. Although 100% specific, it takes six to eight weeks due to slow growth of organisms and further biochemical testing before identification can be made (Heifests, L. B. and Good, R. C. 1994. Current Laboratory Methods for the Diagnosis of Tuberculosis. Tuberculosis Pathogenesis, Protection and Control (ed. B. R. Bloom) ASM Washington DC, pp. 85-110).
Methods based on antigen and antibody detection in body fluids and more recently nucleic acid probes (sequence specific DNA fragments) have been developed as reagents for rapid diagnosis and monitoring of the epidemiology of tuberculosis (Young, D. B. and Mehlert, A. 1989, Serology of Mycobacteria: Characterization of Antigens Recognized by Monoclonal Antibodies. Rev. Infec. Dis. 12:S431-S435; Pfyfer, G. E., Kisling, P., Jahn, E. M. I., Martin, H., Salfinger; W. M. and Weber, R. 1996. Diagnostic Performance of Amplified Mycobacterium Tuberculosis Direct Test with Cerebrospinal Fluid, Other Non Respiratory and Respiratory Specimens, J. Clin. Microbiol. 34:834-841).
The DNA probes utilize a wide array of sequences from Mycobacterium tuberculosis ranging from whole genomic DNA, to a single copy sequence, and to repetitive DNA elements. When evaluated directly on clinical samples they have proved to be highly specific, sensitive and dramatically reduce the time for diagnosis of tuberculosis (Kiehn, T. E. 1993. The Diagnostic Mycobacteriology Laboratory of the 1990's. Clin. Infect. Dis. 17 (suppl.2) S447-S454).
Several sequence specific probes have been used as targets for identification of Mycobacterium tuberculosis by amplification of specific sequences by PCR.
IS6110 is an IS element present in members of Mycobacterium tuberculosis complex (Mycobacterium tuberculosis, M. bovis, M. africanum, and M. microti).
Different regions of IS6110 have been amplified using different sets of primers for PCR based diagnosis like 123 base pair (bp) or 245 bp region (Einsenach, K. D., Cave, M. D., Bates, J. H. and Crawford, J. T. 1990. Polymerase chain reaction amplification of repetitive DNA sequence specific for Mycobacterium tuberculosis. J. Infect. Dis. 161:977-981; Kolk, A. H. J., Schuitema, A. R. J., Kuijper, S., VanLeeuwen J., Hermans, P. W. M., Van Embden, J. D. A. Hartskeerl, R. A. 1992, Detection of Mycobacterium tuberculosis in clinical samples by using polymerase chain reaction and a non radioactive detection system. J. Clin. Microbiol. 30:2567-2575).
IS6110 has some disadvantages. Several Mycobacterium tuberculosis strains with one copy or no copy of IS6110 have been reported (Sahadevan, R., Narayanan, S. Paramsivam, C. N., Prabhakar, R. and Narayanan, P. R. 1995. Restriction fragment length polymorphism typing of clinical isolates of Mycobacterium tuberculosis from patients with pulmonary tuberculosis in Madras, India by use of direct repeat probe. J. Clin. Microbiol. 33:3037-3039: Van Soolingen, D., Dehass, P. E. W., Hermans, P. W. M. Groenen, P. M. A. and Van Embden, J. D. A. 1993. Comparison of various repetitive DNA elements as genetic markers for strain differentiation and epidemiology of Mycobacterium tuberculosis. J. Clin. Microbiol. 31: 1987-1995). Thus the repertoire of Mycobacterium tuberculosis strains present all over the world may not be selected/amplified using a single repetitive element or one DNA probe specific to Mycobacterium tuberculosis. The search for newer DNA probes is a constant requirement.